This invention relates to a process for the separation of C.sub.2+, C.sub.3+ or C.sub.4+ hydrocarbons from a gas stream containing light hydrocarbons and, if desired, of components boiling lower than methane, in which the gas stream, being under an elevated pressure, is cooled, partially condensed and separated into a liquid and a gaseous fraction, and wherein the gaseous fraction is engine expanded (expansion while performing external work--also called work expansion) and the liquid fraction is fractionated by rectification into a product stream containing substantially C.sub.2+, C.sub.3+ or C.sub.4+ hydrocarbons and a residual gas stream containing mostly lower boiling components.
Such processes are particularly useful for the separation of ethane, propane or butane from natural gases or other gases, e.g., refinery gases. Further, these processes are suitable for the separation of similar unsaturated hydrocarbons, i.e., for examples, ethylene, propylene or butylene, provided these components are contained in the gas stream to be fractionated, which is the case for refinery gases. The reprocessing of refinery gases has recently become attractive since market process for LPG (C.sub.3 /C.sub.4 hydrocarbon mixture) have risen, while, on the other hand, vacuum residues and heavy oil are hard to sell. For this reason, the hard to sell heavy products are burned to cover the internal fuel needs of a refinery, while the easily salable C.sub.3+ hydrocarbons are separated from refinery gases especially those which collect in large amounts during processing of light crude oil components into gasoline.
A process of this kind is described in an earlier German patent application No. P 34 08 760.5 filed Mar. 9, 1984 in Germany, having a common assignee, and corresponding substantially to U.S. application Ser. No. 709,742 filed Mar. 8, 1985 by Bauer et al, said U.S. application being incorporated by reference herein.
A major feature of this previously filed U.S. application is that the cold, i.e., refrigeration obtained during engine expansion of the gaseous fraction remaining after partial condensation, is used not for the production of reflux liquid in the rectification column, but for the cooling and partial condensation of the crude gas. Therefore, it is no longer necessary to feed the light components of the gas stream into the rectification column. Elimination of the introduction of the light components present in the feedstock stream (e.g., hydrogen, as well as C.sub.1 and, if desired, C.sub.2 hydrocarbons present in refinery gases; or nitrogen, as well as C.sub.1 and if desired, C.sub.2 hydrocarbons in natural gases) into the separation column makes it possible to perform the rectification at a higher temperature, e.g., about 160.degree. to 300.degree. K., level of the overhead condensor. The possibility of using a simple and inexpensive external refrigeration cycle for cooling of the overhead in the rectification column provide a considerable improvement in carrying out the aforementioned process. Further improvement is nevertheless still desirable.